Mechanisms for the Formation of Active Sites in Single-Atom Alloys

TL;DR

Mechanisms for the Formation of Active Sites in Single-Atom Alloys investigates how reactive dopant adatoms incorporate into inert hosts, focusing on diffusion, attachment, and incorporation pathways on Cu and Ag surfaces across 4d transition metals. Using density functional theory with optB86b-vdW and models of terraces, step edges, and kinks, the authors identify dominant incorporation routes and map periodic trends. They find fast terrace diffusion, barrierless or low-barrier incorporation at steps and kinks, and a U-shaped trend in incorporation energies across the 4d series, with barriers generally rising toward late TMs. They also show how dopant–adatom interactions—repulsive in Pd/Cu and attractive in Ru—modulate diffusion, island formation, and the likelihood of embedding, providing guidance for surface environments that promote SAA formation. This work links fundamental mechanisms to synthesis outcomes, offering a framework for designing SAA catalysts with tailored active-site incorporation.

Abstract

Reactive dopant atoms embedded in inert host metal surfaces define the active sites in single-atom alloys (SAAs), yet SAA synthesis remains challenging. To address this, we elucidate how dopant adatoms deposited on Cu and Ag surfaces become incorporated into the metal and identify periodic trends from early to late transition metals (TMs) using density functional theory. Adatoms diffuse nearly freely across terraces, as diffusion barriers are small, whereas direct incorporation into terraces is unfavourable. In line with conventional wisdom, step edges and kink sites strongly facilitate dopant incorporation, confirming their critical role in alloy formation. Attachment of adatoms to steps and kinks from the lower terrace is favoured. Incorporation then proceeds either from this attached state or when adatoms approach a step edge from above, where reactions often proceed without barrier. Incorporation barriers are generally lower for early and central TMs, increase towards late TMs, and are slightly higher on Cu than on Ag surfaces. Repulsive interactions between Pd adatoms and dopants explain the experimental observation that a dopant-rich brim on the upper terrace of Cu surfaces inhibits incorporation from above. In contrast, attractive interactions, as found for Ru, anchor diffusing adatoms (even on terraces) and promote the formation of adatom islands, yet hinder incorporation next to the dopant and may impede the growth of embedded dopant clusters. By rationalising periodic trends and experimental observations, we show how specific surface sites and adatom--dopant interactions shape dopant incorporation, offering guidance on the surface environments most conducive to SAA synthesis for different dopant elements.

Figures (5)

• Figure 1: Schematic overview of the diffusion and attachment mechanisms considered in this work, together with their periodic trends in reaction energies (triangles) and reaction barriers (diamonds) for 4d TM adatoms on Cu (green) and Ag (purple) surfaces. The left configuration corresponds to the initial reactant, followed by the transition state (if present) and the final product. Illustrated mechanisms are: (i) diffusion of an adatom across a terrace; (ii) hopping of an adatom from the upper to the lower terrace of a step edge; (iii) attachment of an adatom to a step edge from the lower terrace; (iv) diffusion of an adatom already attached to a step edge along the edge; and (v) attachment of an adatom to a kink from the lower terrace. Adatoms are shown in orange and host atoms in grey; atoms in the lower terrace appear in light grey and those in the upper terrace in grey. The surface, step edge, and kink are indicated with a dark grey line. Surface dimensions are for illustrative purposes only; simulation cell sizes are provided in Section S1 of the Supporting Information. Connecting lines are for illustrative purposes only.
• Figure 2: Schematic illustration of the different mechanisms for the incorporation of an adatom as an embedded dopant considered in this work. The left configuration corresponds to the initial reactant, followed by the transition state structure and the final product. The illustrated mechanisms are: (i-a) direct incorporation into a terrace site involving the adatom and one host atom; (i-b) incorporation into a terrace site involving the adatom and a concerted motion of two host atoms; (ii-a) incorporation of an adatom approaching the step edge from above; (ii-b) incorporation of an adatom into the neighbouring row of the step edge, involving the concerted motion of two host atoms; (iii-a) incorporation of an adatom attached to the step edge on the lower terrace, forming a host-atom adatom attached to the upper terrace of the edge; (iii-b) incorporation of an adatom attached to the step edge on the lower terrace, forming a host-atom kink attached to the edge from below; (iv-a) incorporation of an adatom approaching the step edge from above near a kink; (iv-b) incorporation of an adatom approaching the step edge from above at a kink. The adatom and dopant are shown in orange, the moving host metal atoms in blue, and the adjacent host metal atoms in grey. Host atoms in the lower terrace are shown in light grey, while those in the upper terrace are shown in grey. The surface, step edge, and kink are indicated with a dark grey line. Surface dimensions are for illustrative purposes only; the actual simulation cell sizes are provided in Section S1 of the Supporting Information.
• Figure 3: Periodic trends in the reaction energies (triangles) and reaction barriers (diamonds) for incorporation of 4d transition metal dopants on Cu (green) and Ag (purple) host metal surfaces. All energies are given in kJ mol$^{-1}$. While reaction energies exhibit U-shaped periodic trends typical of many SAA properties, reaction barriers display attenuated U-shaped, gradually increasing, or no significant trends across the dopant series, as discussed in the text. Connecting lines are provided for illustrative purposes only.
• Figure 4: Schematic illustration of the two most dominant pathways for the incorporation of adatoms as dopants. Initial deposition of an adatom on a terrace leads to rapid diffusion with small barriers. Assuming a random walk of the adatom, approaches to a defect such as a step edge from above or from below are equally likely. If approaching a step edge from above, incorporation into the step edge is barrierless for most dopant elements and leads to the formation of a host atom kink attached to the edge on the lower terrace, according to mechanism (vi-a). If approaching a step edge from below, the adatom attaches and becomes trapped at the edge until it incorporates following mechanism (iv-b), as the energy required for detachment is typically higher than for incorporation. Schematic energy profiles are shown beneath the structural representations. The adatom and dopant are shown in orange, the moving host metal atoms in blue, and the adjacent host metal atoms in grey. Host atoms in the lowest terrace are shown in light grey, those in the central terrace in grey, and those in the uppermost terrace in dark grey. The surface, step edge, and kink are indicated with dark grey lines. Surface dimensions are for illustrative purposes only; the actual simulation cell sizes are provided in Section S1 of the Supporting Information.
• Figure 5: Schematic illustration of the effects of embedded dopant atoms on the diffusion and incorporation of adatoms. Dopant-promoted island formation: Attractive interactions between adatoms and dopants can immobilise adatoms on the surface, serving as anchor points for the formation of protruding adatom islands or facilitating the formation of embedded dopant clusters. Dopants in brim inhibit step edge approach: Repulsive interactions between adatoms and dopants can suppress incorporation mechanisms when a brim with a high local dopant concentration in the upper terrace next to a step edge prevents diffusing adatoms from approaching the edge from above. Schematic energy profiles are shown beneath the structural representations. The adatom and dopant are shown in orange, the moving host metal atoms in blue, and the adjacent host metal atoms in grey. Host atoms in the lowest terrace are shown in light grey, those in the central terrace in grey, and those in the uppermost terrace in dark grey. The surface, step edge, and kink are indicated with dark grey lines. Surface dimensions are for illustrative purposes only; the actual simulation cell sizes are provided in Section S1 of the Supporting Information.